Wafer inspection systems for inspecting bare or unpatterned wafers are important in many ways, such as qualifying bare wafers, detecting surface anomalies, inspecting rough films, etc. U.S. Pat. No. 6,201,601, issued Mar. 13, 2001, describes a wafer inspection system for bare or unpatterned wafers, made by KLA-Tencor, Inc., generally referred to as the SurfScan system. U.S. Pat. No. 6,201,601 is hereby incorporated by reference in its entirety.
Detectors utilized in wafer inspection systems such as Surfscan include photomultiplier tubes (PMT's). However, as will be described, the desirable characteristics of photomultiplier tubes used in wafer inspection applications differ greatly from those of typical photomultiplier applications, and therefore photomultiplier tubes found in the art are ill suited to wafer inspection.
FIG. 1 illustrates the main features of photomultiplier tubes. Photomultipliers 100 are generally constructed from a glass envelope 102 with a high vacuum inside, which houses a photocathode 110, several dynodes 115, and an anode 120. Incident photons strike the photocathode material, which in transmission mode may be a thin deposit on the entry window of the device, with electrons 105 being produced as a consequence of the photoelectric effect. These electrons are directed by the focusing electrode 125 toward the electron multiplier 130, where electrons are multiplied by the process of secondary emission.
The electron multiplier consists of a number of electrodes called dynodes. Each dynode is held at a more positive voltage than the previous one. The electrons leave the photocathode, having the energy of the incoming photon (minus the work function of the photocathode). As the electrons move toward the first dynode, they are accelerated by the electric field and arrive with much greater energy. Upon striking the first dynode, more low energy electrons are emitted, and these electrons in turn are accelerated toward the second dynode. The geometry of the dynode chain is such that a cascade occurs with an ever-increasing number of electrons being produced at each stage. Finally, the electrons reach the anode, where the accumulation of charge results in a sharp current pulse indicating the arrival of a photon at the photocathode.
Generally, photomultiplier tubes are used in applications which are light-starved, i.e., very high signal gains are desirable. Typically, photomultiplier tubes allow for gains up to one million. In addition, they preferably incorporate photocathodes that respond to a wide variety of wavelengths. Exemplary photocathodes for typical applications may include bi-alkali photocathodes fabricated from a mixture of cesium and potassium. A list of exemplary PMT photocathode materials is found in Photomultiplier Tubes: Basics and Applications (Second Edition), Hamamatsu Photonics, Hamamatsu City, Japan, (1999), pg 34, 54. An example of a photomultiplier tube configuration yielding gains near 1 million is a 9 or more dynode configuration, with secondary emission factors of 6 to 7 per dynode. For example, 96 gives a potential gain of 531,441.